Advanced tape storage devices write data on multiple data tracks on media tapes, which run parallel to each other over the length of the tape. Often times, data is written to the tape in a serpentine format. While using the serpentine format, a tape head of the tape drive begins writing at the start of the tape continues to the end of the length of the tape. Then, the drive steps the head to the next track below, reverses direction, and writes back to the beginning of the length of the tape, and so on. In this manner, the tape head traces a “serpentine-like” figure on the tape as it moves along the tape writing data.
Currently, tape drive systems that guarantee a fixed capacity allocate more recordable media than the fixed capacity requires. The extra recording space is used for write defect skipping. In this way, a system can guarantee, within some probability of defective media events, that the media will hold the advertised capacity.
For linear tape recording systems utilizing serpentine formats that traverse a tape from beginning to end many times, the actual logical end of the tape is at the beginning of the tape closest to its unload point. In this way, minimum unload times for the tape are realized. However, in such a system, when the tape reaches its advertised guaranteed capacity limit, the tape drive ceases to accept more data. A problem arises because the point on the tape where this may occur can be some distance from the beginning of the tape, and therefore unload time for the tape is increased.
For example, one type of media tape may measure 800 meters in length. For a system utilizing a serpentine format, a “wrap” of the tape is known as a single pass of the tape head on the tape before it changes tracks and writes in the opposite direction for another wrap. In some cases, each wrap that runs the length of the tape may hold on the average of approximately 22 Gigabytes (GB) of data. Furthermore, there may be 24 wraps that a tape head will perform while writing data to the tape, resulting in a total writable media space of 528 GB on the tape.
Continuing with the example, assume that the advertised capacity limit of the tape is 500 GB and 500 GB of data is written to the tape without any errors that would require expansion into the extra writing defect space on the tape. In this case, the tape head will stop writing at a point that is about 95% actual capacity of the tape. However, this point is only about two-thirds down the length of the 23rd wrap of the tape, or 533.33 meters away from the physical beginning of the tape.
As a result, when the media drive unloads the tape when it has completed writing, a time delay occurs due to the rewinding of the tape to its physical beginning in order to unload it from the drive. If this problem is magnified over many tapes, then the time delays accumulate and create a significant burden on the tape storage system. A system that reduces the unload time for tapes written in a serpentine format would be beneficial.